Devices employing thermochromic materials for testing the relative charge on a battery are well known. An example of one such device is that commonly found in a package of Duracell.RTM. batteries. This type of device uses a tapered resistance conductor printed on one side of a thin polyester film with a thermochromic material printed in the other side of the film. When a voltage is applied across the tapered resistance conductor, a portion of the conductor roughly proportional to the state of charge of the battery is heated sufficiently to cause a corresponding portion of the thermochromic material to change from opaque to transparent, thus providing an indication of the relative voltage or state of charge of the battery. These devices work well for batteries, such as alkaline batteries, in which the relative voltage varies significantly from a fully charged condition to a depleted condition, for example, 50 percent or from 1.55 volts to 1.0 volts.
However, these tapered resistance devices do not work as well for some other types of batteries where the voltage delivered by a depleted battery is not a significant percent lower than the voltage delivered by a fully charged battery. For example, a lead-acid battery which is typically used in a car, truck or boat exhibits a very small change in voltage with respect to a large decrease in the charge of the battery. For instance, a lead-acid car battery may be at 12.8 volts when fully charged and decrease only to 12 volts when the battery is completely discharged. Rechargeable nickel-cadmium batteries, such as are used in portable computers, video cameras and power tools, also exhibit a relatively small change in voltage with respect to a large change in battery charge. In the case of nickel-cadmium batteries it is also desirable to be able to measure the voltage of the battery accurately in order to ascertain when the battery needs to be recharged. Nickel-cadmium batteries have a memory effect and thus should be almost completely discharged before recharging to promote a complete charge and longer battery life. However, these batteries should not be discharged completely or voltage reversal of the battery can result.
It would be desirable to provide a battery tester which is capable of accurately measuring small changes in the voltage delivered by a battery.